Galvanising process

Galvanising provides outstanding corrosion performance in a wide variety of environments. The galvanising process creates a durable, abrasion-resistant coating of metallic zinc and zinc-iron alloy layers which are bonded metallurgically to the steel and completely covers the item providing a number of significant advantages.

It provides outstanding toughness, resistance to mechanical damage, slows corrosion to about one sixteenth that of steel and a standard minimum coating thickness is applied even to sharp corners to provide a sound and continuous coating.

The galvanising process

Surface Preparation

Preparation is vital to high-quality galvanising. Fero Galv and Fero Blast work together to ensure the preparation for galvanising is perfect. Epoxies, powder coating and other paints must be removed mechanical cleaning such as shot or sand blasting.

Caustic Cleaning

The first cleaning step is caustic cleaning or degreasing in a hot alkali solution to remove contaminates like dirt, grease and oil from the metal surface prior to the galvanising process.

Acid Pickling

Scale, rust, residual paint and other surface contaminates are removed from the steel by acid cleaning or pickling in hydrochloric acids followed by rinsing.

Fluxing

The acid-cleaned steel is then immersed in a flux solution or zinc ammonium chloride and wetting agents to remove the oxide film, which forms on highly reactive steel surfaces following acid cleaning and prevents further oxidisation. This process also heats the steel to between 60-80 degrees Celsius to prepare it for the high temperatures of hot-dipping in the zinc bath.

Hot-dip galvanising

The molten zinc is heated to about 450 degrees Celsius. When the steel is immersed in the galvanising bath at a controlled rate, the steel surface is coated by the molten zinc resulting in a reaction between the zinc and the formation of a series of zinc-alloy layers. This process takes about 10-15 mins, longer for larger items, and the resulting zinc-alloy layers are actually harder than the base steel. As the item is removed, again at a controlled rate, the molten zinc solidifies to form the outer zinc coating.

Quenching

After galvanising, the steelwork is immediately dipped is a quench solution which contains additives to prevent the formation of wet storage staining or "white rust" occurring. This process also cools the steelwork in order to facilitate the efficient movement of steel products. Some products can be air-cooled if required.

Fettling

Then any remaining excess drips and drags are removed.

Galvanising small components (spinning)

Small components are loaded into baskets for galvanising. Once removed from the molten zinc the spinning baskets are centrifuge spun to remove excess zinc.

Coating thickness

The total zinc coating mass or coating thickness of galvanized steel depends mainly on the mass and thickness of the steel being galvanised.

Other factors influencing coating thickness include:

Surface condition:

The process of grit blasting steel before galvanising increases the surface area and results in greater zinc-alloy growth during galvanising, producing a thicker coating, but also a rougher finish of the surface.

Composition of steel:

Silicon and phosphorous content can have a major effect on the structure, appearance and properties of galvanised coatings.

Silicon:

Certain levels of silicon content will result in excessively thick galvanised coatings. Steels with silicon content in the range of 0.04-0.14 per cent result in excessive growth of zinc-iron alloys on steel surfaces and will generally have a dull grey appearance. Growth rates are less for steels containing between 0.15 and 0.22 per cent and increase again with greater silicon levels.

Phosphorous

The presence of phosphorous above 0.5 per cent produces an increase in the coating growth. When present in combination with silicon excessively thick galvanised coatings can be produced.